This invention relates to improved techniques for conducting well logging operations and more particularly to a method for improving the results of thermal neutron decay time or neutron lifetime well logging by the employment of well cement having a high thermal neutron capture cross section.
In recent times the logging of cased well boreholes has become an important function in the petroleum producing industry. These logging operations have become important due to the increased amount of well work over activity in old wells, and in new wells because of the precision with which measurements may be made which relate to the earth formation water and oil saturations through the use of nuclear logging instruments. In particular, the thermal neutron lifetime or thermal neutron decay time log has become important in the logging of cased well boreholes due to the fact that this instrument can measure the thermal neutron macroscopic capture cross-section of the earth formations. This macroscopic capture cross-section (if the earth formation porosity is known) may be used to determine directly the water saturation, Sw, and hence to infer the oil saturation, of formations adjacent to a cased well borehole.
Nuclear well logging parameters such as the thermal neutron decay time are particularly important because in a cased well borehole it is difficult to obtain other more conventional types of well logs such as resistivity logs, which are shorted out by the steel casing which separates the borehole from the earth formations surrounding it. Thus in some instances the thermal neutron decay time or neutron lifetime logs may serve to be the most effective instrument for investigating earth formations in the vicinity of a caed well borehole.
Commercially available techniques for performing thermal neutron lifetime or thermal neutron decay time measurements in the vicinity of a cased wellbore have generally assumed that the thermal neutron lifetime or thermal neutron decay time of the borehole fluid immediately surrounding the detector used in such a system is substantially shorter than that of the earth formations in the vicinity of the borehole. Under such conditions the measurements made in typical cased well boreholes can accurately reflect the macroscopic thermal neutron capture cross-section of the earth formation surrounding the borehole. In laboratory studies of the depth of investigation of such systems however, it has been found that the thickness of the cement sheath surrounding the well casing can effect the measurement. In particular, it has been found that if this cement sheath has a relatively low macroscopic thermal neutron capture cross-section, the effective region of investigation of such an instrument is drastically reduced to that interval very near the borehole. It has also been found that variations in the thickness of the cement sheath under these conditions can deteriorate the quality of the measurement of thermal neutron lifetime or thermal neutron decay time of the earth formation materials in the vicinity of the borehole to the point where the formation materials may only contribute small amounts to the total measurement.